Foundation, Concrete and Earthquake Engineering

What is Driving Energy? Its Significance in Standard Penetration Test (SPT)

The purpose of standard penetration test is to determine penetration resistance of sampler and to obtain representative soil sample. Two important things should keep in mind that the sampler is split-barrel sampler and sampling method yields disturbed soil sample. Thus this test is conducted by driving (into the ground) a split-barrel sampler to collect disturbed soil sample that represent corresponding layer and to measure resistance rendered to penetrate through the stratum by that sampler.

Center-hole or Donut hammer for standard penetration test
Center-hole or Donut hammer for 
standard penetration test
This sampling procedure and sample collected in this test method are not our concern in this article; we will mainly discuss about driving resistance. Driving resistance is related to driving energy which is exerted by falling a driving mass on drilling head. For convenient of discussion, at first, it is required to include some terminology about drilling equipment and test procedure.

Drilling rods-Rods through which downward force is exerted to advance drill bit with necessary torque into the borehole. 

Drive weight assemblage-This include a hammer, anvil, a guide system for falling hammer, drill rod and necessary hoisting attachments.

Drill head or anvil-This is an essential part of drive-weight assemblage on which falling hammer strikes. It is a media to transmit hammer energy into drill rods. 

Hammer-This is the important part of drive weight assemblage which exert impact weight of 623±9N (140±2 lbf) by falling from specified height. This energy is used to penetrate hole and to make sample for field or laboratory. 

Hammer fall guide-This term is self-explanatory i.e. guide the falling of the hammer.

Sampling rods-Rods connecting drive-weight assemblage to the sampler.

Cathead-The rotating drum like windlass over which the rope pass back and forth. This is a lifting system around which a rope is wrapped by operator to lift and successive dropping of hammer by tightening and loosing of the rope with counterclockwise and clockwise rotation of drum.

N-value - This represents number of blow as an indication of resistance of soil to be tested against penetration. The N-value is blow counted to penetrate each foot on the ground which is a summation of blows counted to drive the split-barrel sampler up to 150 mm to 450 mm (6 to 18 inches) depth from where the sampler is seated.   

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